In recent years, there is a need to develop automobiles that have low environmental impact and increased safety. In a world of expanding globalization, car makers have advanced development of light and strong vehicle bodies as well as low emission engines and improved air bags. Automobile lightening has been progressed by reducing thickness of steel components without decreasing their strength. From the mid-1990s, a high tensile strength steel sheet that can be used as a metallic material which contributes to the lightening of an automobile has been a focus of attention.
Generally, strength of a steel sheet is expressed in tensile strength. A steel sheet having tensile strength of 340 N/mm2 or more is referred to as a high tensile strength steel sheet. Conversely, a steel sheet having tensile strength of 280 N/mm2 or less is referred to as a mild steel sheet. Recently, the high tensile strength steel sheet having a tensile strength of 1000 N/mm2 or more has been used to enhance collision safety.
The high tensile strength steel sheet may also be referred to as “high strength steel sheet” or “high tensile material.” However, Japanese Industrial Standard generally uses the term “high tensile strength steel sheet” in, for example, JIS G 3134 (which is directed to “processable hot rolled high tensile strength steel sheets and bands for mobile application”) and JIS G 3135 (which is directed to “processable cold rolled high tensile strength steel sheets and bands for mobile application”). Therefore, in this description, “high tensile strength steel sheet” will be used hereinafter.
Generally, the high tensile strength steel sheet has increased intensity and increased yield strength. As a result, the high tensile strength steel sheet has reduced ductility, which is caused by the increased intensity. The reduced ductility may cause poor formability. Also, the high yield strength may inherently provide high spring back performance. Such high spring back performance may produce a number of defects in a product that is formed from the high tensile strength steel sheet by press forming. Such defects in the press formed product may include surface distortion, bad shape stability, cracking, reduced accuracy and galling. Thus, in order to reliably press form the high tensile strength steel sheet, there are a number of technical problems to be solved.
Recently, when a metallic material or steel sheet is processed (i.e., press formed), lubricants are generally omitted in order to reduce processing costs. In addition, after the steel sheet is processed, rust inhibitive oils are generally omitted. Therefore, if the steel sheet is press formed without using the lubricants, the steel sheet cannot be suitably press formed because of lack of lubricity, thereby producing cracking and galling in a formed product. Also, such lack of lubricity may increase friction between the steel sheet and forming dies. Such friction may significantly reduce service life of the forming dies.
In order to solve these problems, there is a need to develop lubricants or rust inhibitors that provide excellent lubricity during the press forming of the steel sheet. Up to now some special lubricants have been developed. For example, Japanese Laid-open Patent Publication Number 10-279979 teaches a rust inhibitive oil solution for use in the press forming of the steel sheet. This oil solution contains a rust inhibitive agent, ultrabasic calcium sulfonate, a sulfuric extreme pressure agent and potassium borate. However, this oil solution contains a boron compound (potassium borate) that is pertinent to Pollutant Release and Transfer Register (PRTR). Therefore, such an oil solution is negative from the viewpoint of environmental preservation. Also, Japanese Patent Publication Number 7-42470 or Japanese Laid-open Patent Publication Number 8-311476 teaches a rust inhibitive oil solution having kinetic viscosity of 40 mm2/s or less at 40° C. This oil solution may have excellent rust inhibiting performance and self-removing performance. However, this oil solution has less lubricity. Therefore, such an oil solution is not suitable for processing (press forming) the high tensile strength steel sheet because the high tensile strength steel sheet may be subjected to extremely large stress.
Post-treatment of the press forming may, for example, include the steps of (1) degreasing and washing a formed product in order to remove lubricants, (2) applying the washed product with rust inhibitive oils in order to protect the product from rusting, (3) plating or coating the product, (4) treating the product by heat in order to strengthen the product, and (5) welding the product to another metal component.
In order to weld the high tensile strength steel sheet, a metal active gas (MAG) welding method using gaseous carbon dioxide (CO2) as a shielding gas is often used. This welding method is one of many steel welding methods and is referred to as a CO2-MAG welding method. The CO2-MAG welding method is the most widely used arc welding method for welding steel. For example, the CO2-MAG welding method is commonly used in many industries of, for example, pressure containers, bridge frames, constructional steel frames, ships, marine structures, heavy machinery, chemical plants, nuclear plants, motorcycles and automobiles. Generally, the CO2-MAG welding method has advantages of increased welding speed, high welding efficiency and easy handling. Also, this welding method may provide high quality welding portions. Further, this welding method can be applied to metallic materials having a wide variety of thickness without changing a welding wire.
In the CO2-MAG welding method, it is possible to use a pure gas of carbon dioxide and a mixed gas of argon and carbon dioxide as the shielding gas. However, the pure gas of carbon dioxide is a highly oxidized gas. Such an oxidized gas can oxidize and deteriorate a welding product (i.e., a welding composite constituted of a welding wire metal and a matrix steel) produced in the welding portions because the welding portions can be heated to about 1500° C. (i.e., a melting point of the welding wire) or more. The deteriorated welding product may reduce bonding strength of the welding portions. Therefore, when the CO2-MAG welding method is used for welding the high tensile strength steel sheet, the mixed gas of argon and carbon dioxide may generally be used as the shielding gas in order to prevent the welding portions from excessively deteriorating. Preferably, the mixing ratio of argon to carbon dioxide is approximately 80:20.
Further, according to Japanese Industrial Standard, the CO2-MAG welding method is simply referred to as a “MAG welding method” regardless of whether the shielding gas is the carbon dioxide pure gas or the argon-carbon dioxide mixed gas. Therefore, in order to mention the CO2-MAG welding method, the “MAG welding method” will be used here on a nonexclusive basis. That is, herein, the “MAG welding method” will refer to both of the CO2-MAG welding methods in which the carbon dioxide pure gas and the argon-carbon dioxide mixed gas are respectively used as the shielding gas.
In addition, when the high tensile strength steel sheet is welded by the MAG welding method, the MAG welding method is sometimes performed without removing the lubricants from the high tensile strength steel sheet. In such a case, the lubricants may decompose, thereby producing corrosive compounds. The produced corrosive compounds may produce corrosion on the welded product (i.e., weldment). The corrosion thus produced may deteriorate the weldment in quality.
Further, the lubricants must be removed from the press formed and welded product before the product is plated. Therefore, it is essential that the lubricants can be easily removed or washed out from the product.